1. Field of the Invention
The present invention relates to a light emitting device package and a method for manufacturing the same, and more particularly, to a light emitting device package capable of achieving an enhancement in light emission efficiency and a reduction in thermal resistance, and a method for manufacturing the same.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are well known as a semiconductor light emitting device which converts current to light, to emit light. Since a red LED using GaAsP compound semiconductor was commercially available in 1962, it has been used, together with a GaP:N-based green LED, as a light source in electronic apparatuses, for image display.
The wavelength of light emitted from such an LED depends on the semiconductor material used to fabricate the LED. This is because the wavelength of the emitted light depends on the band gap of the semiconductor material representing energy difference between valence-band electrons and conduction-band electrons.
Gallium nitride (GaN) compound semiconductor has been highlighted in the field of high-power electronic devices because it exhibits a high thermal stability and a wide band gap of 0.8 to 6.2 eV. One of the reasons why GaN compound semiconductor has been highlighted is that it is possible to fabricate a semiconductor layer capable of emitting green, blue, or white light, using GaN in combination with other elements, for example, indium (In), aluminum (Al), etc.
Thus, it is possible to adjust the wavelength of light to be emitted, using GaN in combination with other appropriate elements. Accordingly, where GaN is used, it is possible to appropriately determine the materials of a desired LED in accordance with the characteristics of the apparatus to which the LED is applied. For example, it is possible to fabricate a blue LED useful for optical recording or a white LED to replace a glow lamp.
On the other hand, initially-developed green LEDs were fabricated using GaP. Since GaP is an indirect transition material causing a degradation in efficiency, the green LEDs fabricated using this material cannot practically produce light of pure green. By virtue of the recent success of growth of an InGaN thin film, however, it has been possible to fabricate a high-luminescent green LED.
By virtue of the above-mentioned advantages and other advantages of GaN-based LEDs, the GaN-based LED market has rapidly grown. Also, techniques associated with GaN-based electro-optic devices have rapidly developed since the GaN-based LEDs became commercially available in 1994.
GaN-based LEDs have been developed to exhibit light emission efficiency superior over that of glow lamps. Currently, the efficiency of GaN-based LEDs is substantially equal to that of fluorescent lamps. Thus, it is expected that the GaN-based LED market will grow significantly.
By virtue of such technical development, the application of GaN-based LEDs has been extended not only to display devices, but also to an LED backlight substituted for a cold cathode fluorescent lamp (CCFL) used for a backlight of a liquid crystal display (LCD) device, a white LED lighting device usable as a substitute for a fluorescent lamp or a grow lamp, and signal lamp.
Meanwhile, in addition to LEDs driven by DC power, high-voltage AC LED chips, which can be driven even by general AC power, have also been developed. For such an application, LEDs should exhibit a high operating voltage, a small drive current, a high light emission efficiency, and a high brightness at the same electric power.
Referring to FIG. 1, a structure of a general LED is illustrated. As shown in FIG. 1, a buffer layer 2, an n-type semiconductor layer 3, an active layer 4, and a p-type semiconductor layer 5 are sequentially deposited over a substrate 1 made of, for example, sapphire. Mesa patterning is then performed such that the n-type semiconductor layer 3 is exposed. Thereafter, a current diffusion layer 6 is formed on the p-type semiconductor layer 5, as a transparent electrode having a high light transmissivity.
For electrical connection of the LED to an external circuit, a p-type electrode 7 and an n-type electrode 8 are subsequently formed over the p-type semiconductor layer 5 and n-type semiconductor layer 3, respectively. Thus, an LED structure 10 is completely formed.
When a voltage from the external circuit is applied between the p-type electrode 7 and the n-type electrode 8 in the LED, holes and electrons enter the p-type electrode 7 and n-type electrode 8, respectively. The holes and electrons are re-coupled in the active layer 4, so that surplus energy is converted into light which is, in turn, externally emitted through the transparent electrode and substrate.
At this time, static electricity and a surge voltage may be applied to the p-type electrode 7 and n-type electrode 8 electrically connected to the external circuit, so that overcurrent may flow through the LED structure 10. In this case, the semiconductor is damaged, so that the LED can be no longer used.
In order to solve this problem, a voltage regulator is electrically connected to the LED. When overcurrent is generated, the voltage regulator bypasses the generated overcurrent, thereby preventing damage of the LED chip.
For such a voltage regulator, a zener diode using zener breakdown is mainly used. When a diode is fabricated to have a very high impurity concentration, it has a space charge region width. In this case, a strong electric field is generated even at a small reverse voltage.
The strong electric field generated as above releases covalent bonds of a lattice, thereby producing a number of free electrons and a number of free holes. As a result, an abrupt reverse current flows under the condition in which there is little voltage variation. In accordance with such a zener diode function, it is possible to prevent damage of the LED chip.
In an example of a conventional package using such a zener diode, a cup-shaped curved portion is formed at a lead frame, and an LED is bonded to the curved portion of the lead frame. In this case, a voltage regulator such as a zener diode is bonded to another lead frame of the package. The lead frames are then wire-bonded to connect the voltage regulator and LED in parallel.
In the above-mentioned conventional method, there may be a degradation in electrical and optical characteristics and an increase in costs because it is necessary to form the cup-shaped curved portion, and to connect the voltage regulator, which is separately prepared, using an off chip method.